The invention relates to a method of treating a fiber web, in which method the fiber web is dried substantially dry and after that led to be treated by means of heat and compression.
The invention relates further to an arrangement for treating a fiber web, which arrangement comprises a drying apparatus for drying the fiber web substantially dry and means for treating the substantially dry fiber web by means of heat and compression.
If desired, a dried fiber web can be led to be calendered for polishing and smoothing the surface of the fiber web and for equalizing variations in web thickness. When calendering the fiber web, it is led through one or several compression zones or nips formed by rolls having a hard and smooth surface. The rolls can be heated, whereby the web is exposed, besides to compression, also to a heat treatment. The properties of the fiber web obtained from calendering depend very much on the surface pressure of the compression zone and on the number of nips. To provide a sufficient effect, many nips are often needed, which results in a structure comprising several rolls often mounted on each other. Such a structure is high and very inconvenient. Moreover, web conveying to the calender and from the calender forward in the process is difficult and complicated, because web threadings take place over long open gaps between these different parts of the process. In this way, the surface smoothness and gloss of the fiber web can be improved, but a treatment of this kind does not provide a sufficiently good final result in all cases, however.
The object of this invention is to provide a method and an arrangement, by means of which the surface treatment of the fiber web is well- and easily-implemented and can be included in a simple manner directly in the web making process, if desired.
The method of the invention is characterised in that at least one surface of the dry fiber web is arranged moist and led to an apparatus comprising two endless air-impermeable bands, first turning rolls, the first band being arranged to turn around the first turning rolls, and second turning rolls, the second band being arranged to turn around the second turning rolls, whereby the first band and the second band are arranged to run part of the way parallel to each other and whereby at least one band is heated.
Further, the arrangement of the invention is characterised in that the arrangement comprises at least one moistening apparatus for moistening at least one surface of the fiber web and an apparatus comprising two endless air-impermeable bands, first turning rolls, the first band being arranged to turn around the first turning rolls, and second turning rolls, the second band being arranged to turnaround the second turning rolls, whereby the first band and the second band are arranged to run part of the way parallel to each other, whereby at least one band is arranged to be heated by heating means and whereby the fiber web is led between the bands.
The essential idea of the invention is that a fiber web is dried substantially dry and led after that between two parallel bands moving in the same direction, whereby at least one of the bands is heated. Before the fiber web is led between the bands, the fiber web surface to be arranged against the heated band is moistened or the fiber web is dried one-sidedly in such a way that the moist or moistened surface of the fiber web bears against the heated band. The idea of a preferred embodiment is that the other band is cooled and a felt or wire is arranged particularly preferably between the fiber web and the cooled band.
An advantage of the invention is that the gloss and smoothness of the fiber web, the density of the web surface and/or some other property of the surface can be made extremely good. Further, by drying the fiber web throughout dry and by moistening the web surface to be arranged against the heated band, the moistening can be implemented in a controlled manner and the moisture profile of the fiber web can be made even. By cooling the other band, and especially, by arranging a felt or wire between the fiber web and the cooled band, it is possible to use for the treatment larger amounts of water, which can be removed at the treatment by means of the felt or wire and the cooled band, and the fiber web can be provided with a very good smoothness and gloss. Additionally, the apparatus constituted by said bands can easily be located in the process in such a way that no large open gaps remain in the travel direction of the web and no special web threading arrangements are needed. Moreover, in the solution of the invention, the risk of web breaks is small, because the fiber web is between the bands. This eliminates the roll damaging due to web breaks or the like occurring frequently in conventional calenders. The efficient treatment of the web surface according to the invention provides a more permanent smoothness, enduring for instance later moistening phases of printing. Moreover, the need of energy is reasonably small. In the solution of the invention, the smoothening surfaces are even more durable than in the conventional calendering method.